Bipod Instrumentation

Introduction

Storm activity is often associated with erosion of the
subaerial beachface (List and Farris, 1999) and inner surf zone. These same
storms may also lead to erosion or accretion deeper on continental shelves due
to exchange of sediments between onshore and offshore locations. Even with this
sediment exchange, offshore decreases in profile variability (Nicholls et al.,
1998) suggest that the inner continental shelf is responding to waves and
currents at different temporal scales than the subaerial beachface and inner
surf zone.

Many studies have concentrated on the forces which can
initiate and sustain sediment transport on continental shelves, but the actual
amplitudes and nature of seabed responses to storm events are not well
constrained (Morton, 1988). In particular, processes controlling scour and
creation of marine erosion surfaces are not well documented (Field et al.,
1999).

With advances in technology, longer-term observations of
seabed dynamics have the potential to increase our understanding of seabed
elevation response to different types of storm events (Beavers et al., 1999).
Field measurements of seabed elevation changes during northeaster storms
(Wright et al., 1994a) and hurricanes (Beavers et al., 1999) have been
documented on the inner continental shelf, but rarely have both types of storm
events been documented at the same location on the shelf.

By maintaining instrument packages at the same location
for several years (1994-1997), temporal patterns in seabed response and the
spatial variability of hydrodynamic forcing can be studied for a variety of
storms. In order to obtain continuous seabed observations and document
hydrodynamic conditions throughout storm events, instrument packages were
deployed in 5.5, 8, and 13 m water depths beginning in 1994. Designed to span
the transition from the inner continental shelf to the outer surf zone, these
packages occupy a dynamic zone where both wind and wave forcing may be
important (Fig. 2.1). The 2 major storm systems responsible for producing this
wind and wave forcing at Duck, NC, are hurricanes and northeaster
storms.

Bipod Instrumentation

To study longer-term sediment dynamics on the inner
continental shelf and outer surf zone, a multi-year monitoring program of
near-bottom and interior flows and seabed elevation changes across the
shoreface of the FRF was initiated in 1994 (Howd et al., 1994). Instrument
packages to monitor waves, currents, and seabed elevation changes were deployed
in 5.5 and 13 m water depths in September and October 1994 (Fig. 2.3). In May
1995, a third instrument package was deployed in 8 m water depth.

Figure 2.3. Location of
bipod instrumentation (stars) at the FRF. Contours are in meters.

Instrument packages were secured on bipod
frames (Fig. 2.4) designed to sleeve over two 6.4 m long pipes jetted
vertically 4 m into the seabed. Power and communications were provided from
shore via armored multi-conductor cables. Except for sensor repairs or
replacement, these instrument packages collected data during numerous storms
from 1994-1997.

Figure 2.4. Bipod
instrumentation.

Current Meters

Each bipod (Fig. 2.4) initially included 3 Marsh-McBirney
electromagnetic current meters located on the offshore end of the frame. The
biaxial electromagnetic current meters were replaced in fall 1997 with
non-invasive triaxial acoustic current meters. This end of the frame was
deployed to the southeast to minimize interference of current meters and
vertical support with wave orbital velocities during northeast waves. Current
meters were initially deployed at nominal elevations of 0.2, 0.55, and 1.5 m
above the seabed to permit calculation of bed shear stresses associated with
different flows by the velocity profile method (Drake and Cacchione, 1992).
With a shoreline orientation of approximately N20W, longshore currents flow
toward 340° (i.e.
northward) or toward 160° (i.e.
southward). Similarly, cross-shore currents are either onshore at 250°
(westward) or offshore at 70° (eastward).

Pressure sensor

A pressure sensor (Fig. 2.4, P), sonar altimeter (S), and
electronics housings (A, B, and C) were secured to the frame crossbeams.
Current meters and Sensometric strain gauges were sampled at 2 Hz. Pressure
fluctuations were measured to allow calculation of the wave spectrum and water
elevation (tides). Initially, an analog Sensometric strain gauge (Fig. 2.4, P)
was deployed with each instrument package. These sensors were relatively
inexpensive and reliable, but often exhibit mean pressure drifts over long time
periods, such as 10-20 cm in a month. In September 1997, digital Paroscientific
gauges replaced the strain gauges for more precise and stable pressure
measurements. These gauges output voltage signal with a frequency proportional
to the pressure and operate at a nominal 38 kHz. The Tattletale Model 8
operated in a frequency-count mode to measure the Paroscientific signal over a
50 ms averaging interval, at a 2 Hz rate. This sample interval was determined
to be short enough to have negligible filtering effect for wave measurements
(2+ s), and long enough for an accurate pressure (frequency) measurement of
better than 1 mm.

Wave height, Hmo, was computed as an
energy-based statistic equal to four times the standard deviation of the sea
surface elevations. Wave height reported from the pressure gauge has been
compensated for hydrodynamic attenuation using linear wave theory. Wave
variance is computed from energy spectra and band limited to frequencies >
0.05 Hz (period <20 s) with a high frequency cutoff based on wave
attenuation where linear theory amplitude correction is 10. Wave period is
identified from the computation of a variance (energy) spectrum with 60 degrees
of freedom calculated from a 34 minute record. Peak wave period, Tp,
is defined as the period associated with the maximum energy in the
spectrum.

Sonar altimeter

The Datasonics altimeter (Fig. 2.4, S) transmits a 210
kHz acoustic pulse once per second (1 Hz) with bottom return echoes
detected after each pulse. Returns are range-binned for 34 minutes. The bin
with the maximum number of returns is recorded as the seabed elevation during
that 34-minute period.In laboratory tests, the mean distance to the bottom
(Fig. 2.4, d) measured with the altimeter was accurate to + 1 cm of an
independent distance measurement. The altimeter transducer beamwidth is
approximately 10° and
results in an approximately 20 cm diameter footprint at 1 m range. The
footprint of the sonar altimeter is too large to resolve short wavelength (1-5
cm) ripples (Gallagher et al., 1996); instead, larger scale patterns of erosion
and deposition are resolved.